This is the 2nd blog in a series about research and conservation activity in the Upper Henry’s Fork, focusing on Island Park Reservoir. Click here for part 1 and here for part 3.
Island Park Reservoir is not like other Reservoirs
Unlike other reservoirs, Island Park Reservoir is shallow, warm, and nutrient-rich, with lots of sediment and algae that reduces water clarity. That isn’t necessarily a bad thing, unless things go too far.
Island Park Reservoir’s most unique feature is its thermal properties (how it absorbs and retains heat). This is important since the thermal properties of lakes and the annual circulation events they cause have huge consequences for a lake’s condition. Lake water absorbs heat energy directly from sunlight and additional heat from the air.
Water density refers to how closely packed together water molecules are (in essence, the weight of a specific volume of water). Water density changes with temperature. In spring, as the water at the surface warms up, it becomes lighter (or less dense) than the cooler water at the bottom of the lake. This is why ice cubes will sink in water...because ice cubes are heavier than the warmer water in the glass. When the difference in density ("weight") between the surface and bottom layers of a lake become too great for the wind to mix them, the lake is said to be “stratified.”
“Thermal stratification” is the term used to describe this situation in most lakes, where warm water sits on top of colder water. These two layers don’t mix until fall, when the temperatures level out - a process known as “turnover.”
Island Park Reservoir can be thought of as having three distinct layers
Eastern Surface: Consists of a warm surface layer of water.
Eastern Bottom: Consists of a cool bottom layer of water.
Western Section: The western two-thirds of the reservoir (especially west of Bill's Island) is too shallow and wide to form these layers of warm and cool water, and so it remains well-mixed.
Note: Most of the incoming and outgoing water is limited to the eastern third section of the reservoir. The water temperature and density between the Western and Eastern segments of the reservoir are different.
Density Currents
The independent western part of Island Park Reservoir is uniquely sensitive to wind and weather. This part of Island Park Reservoir is relatively shallow, with an average depth of about 12 feet. A shallow depth and relatively small volume given its large surface area means the water temperatures, dissolved oxygen, and clarity can change rapidly with weather conditions. Underneath that shallow water is the dirt and vegetation that used to make up Shotgun valley. Much of this dirt is fine, wind-blown sediment deposited in what was once Shotgun Valley over the course of thousands of years. A strong wind or excessive recreational boating can stir up the lake bottom. Under the right circumstances, which include high drawdown, low inflow, and during cold fronts, this turbid water can flow towards the dam and exit the reservoir into the river downstream in what we call a “density current”.
Other Factors that Come into Play
Island Park Reservoir’s condition is uniquely sensitive to its watershed, including human activity such as development, environmental changes, the natural geology, and weather. The Henry's Fork Watershed is large (482 square miles) when compared to the total lake size of Island Park Reservoir (~7800 surface acres). That means there’s about 40 acres of land area draining into Island Park Reservoir for every acre of lake.
Island Park Reservoir also has a relatively small total volume (135,205 acre-feet) relative to normal annual inflow (302,878 acre-feet). That’s a lot of water coming off the landscape and into the reservoir! What happens on land is certainly going to affect the quality of water flowing into Island Park Reservoir. That land is rapidly developing, undergoing changes due to a changing climate, and is naturally rich in nutrients like Phosphorus.
Nutrient inputs from the landscape to Island Park Reservoir are increasing, and contribute to Island Park Reservoir’s high nutrient concentrations. Other sources of nutrients are from sediment resuspension and a phenomenon called “nutrient recycling”. Nutrient inputs to Island Park Reservoir from all three sources are likely increasing. For example, our data show a doubling of nutrient concentrations in the Henry’s Fork upstream of Island Park Reservoir over the past 10 years and a resulting increase in algal concentrations, especially around the Henry’s Fork inlet. Sediment resuspension on the western two thirds likely drives algal blooms there.
High nutrient concentrations are not automatically a bad thing. Nutrients like phosphorus and nitrogen are necessary for the growth of algae and rooted aquatic vegetation. The right amount of algae and plants form the base of the food web and places for fish to hide.
However, too many nutrients and too much algae can cause a detrimental phenomenon called “nutrient recycling”. Lakes with excessive algal blooms have low water clarity. When water clarity is low, algae and plants cannot get enough sunlight, and so they sink to the bottom and die. Decomposition of dead algal and plant matter suppresses oxygen in the lower layer of water and can even make its way into the Henry’s Fork downstream. The loss of oxygen hurts fish habitat and also creates chemical reactions that recycle more nutrients into the water column. This fuels a new algal bloom, which further reduces water clarity. This nutrient recycling is one of the likely culprits behind poor fish habitat and recreational closures due to toxic algae in Island Park Reservoir.
What about Water Temperatures?
Water temperatures are high in Island Park Reservoir because of its shallow average depth and low volume. With much of the reservoir exposed to the atmosphere and baking under the sun, water temperatures can reach 75 F or more. Like with nutrients, this warm water is not automatically a bad thing.
When the cool bottom layer is well-oxygenated and available for fish, there is plenty of habitat for fish to survive the summer and plenty of good-quality water flowing into the Henry’s Fork downstream. But as we know, high temperatures become a problem when the bottom layers of water are too low in oxygen (due to excessive algae decomposition). We also already know that water temperatures in Island Park Reservoir and the Henry’s Fork River are higher when Island Park Reservoir drawdown increases.
Check out the third blog post in this series to learn about how HFF can improve water quality at Island Park Reservoir!
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